The present invention relates to stretch-forming of tubes, particularly of long tubes with accurately defined inner and outer diameters, i.e. these diameters are to be within predetermined tolerances upon completion of the stretch-forming process and the surfaces should have a rather fine finish even without a separate finishing step.
It is known to stretch-form short tube sections by means of a sleeve-like drawing die of accurate dimensions. This way, one obtains inner and outer dimensions within very low tolerances, and the inner surface is sufficiently smooth so that additional finishing may not be required. Furthermore, the texture of the material is improved by the stretch-forming operation. The known methods fail, however, when applied to longer tubes. This is particularly due to the requirement for a drawing die having the length of the completed tube and having the required surface finish. Accurate deep drilling is not possible for lengths exceeding, say 15 feet.